Hezbollah is rapidly rearming in preparation for a new conflict with Israel, fearing that Benjamin Netanyahu's government will attack Lebanon again prior to any assault on Iran's nuclear facilities.

Last week, Israeli commandos seized a ship in the Mediterranean loaded with almost 400 tonnes of rockets and small arms – which Israel claimed was being sent from Iran to its Hezbollah allies. In dramatic further evidence of growing tensions, the Observer has learned that Hezbollah fighters have been busy reinforcing fixed defence positions north of the Litani river.

Having lost many of its bunkers in the south, Hezbollah is preparing a new strategy to defend villages there.

Although the organisation denied last week that the weapons were intended for its use, senior commanders have done little to disguise the scale of rearmament. "Sure, we are rearming, we have even said that we have far more rockets and missiles than we did in 2006," said a Hezbollah commander, speaking on condition of anonymity.

The 2006 war between Hezbollah and Israel began after an ill-advised operation by to kidnap two Israeli soldiers, prompting a massive Israeli response that lasted 34 days and killed more than 1,000 people.

"We had to blow up or leave some of our bunkers and fighting positions, but we still have plenty of capabilities in the south. We expect the Israelis to come soon, if not this winter, then they will wait until spring, when the ground isn't too soft for their tanks."

It was expected that the ceasefire would neutralise Hezbollah military efforts along the Lebanon-Israel border, as a newly bolstered United Nations peacekeeping force and the Lebanese army took up positions. Instead, based on dozens of interviews and multiple trips into the country's south, it is clear that Hezbollah believes it would face different challenges.

It has been forced to abandon the line of deeply entrenched static positions on the border with Israel and withdraw most of its men and weaponry to clusters of Shia villages.

"It's clear that Hezbollah no longer controls the border, due to the presence of Unifil [United Nations Interim Force in Lebanon] troops," said Andrew Exum, a military expert on Hezbollah at the Centre for New American Security. "They appear to be hardening the villages for this next round of fighting, while pushing their fixed positions north away from Unifil to protect the approaches to Beirut and the Bekaa Valley."

Israel and the United States have long assumed that any military action against Iran's nuclear programme would draw a muscular response from its close allies in Hezbollah. According to Israeli military and intelligence analysts, any move against Iran would require a move first against Hezbollah's capability to disrupt life in northern Israel with its rockets.

Tel Aviv seems unlikely to commit the same mistakes it did in 2006, when the plan was for air strikes to disrupt and confuse Hezbollah's military command, while minimising the use of ground troops. Israeli military sources have said that they are preparing for a potential new conflict.

Cruising through the serene green wadis that connect south Lebanon to the Litani river to the north, the commander explains what happened at the end of the last war. "We knocked out three of their tanks on the first day, as they tried to enter," he explained at a turn-off by the village of al-Qantara. "But after they entered the wadi, we knew they were going for the river and had to be stopped. So we called out to all the special forces anti-tank teams in the area. And they all swarmed the wadi. Boys would set up and wait for the tanks, fire off their rounds and then pull back. Then they would pull back a kilometre or so down the wadi and wait for them again."

According to Israeli military reports, after the first and last tanks were hit by rocket fire or mines, killing the company commander, the 24 tanks were essentially trapped inside a valley, surrounded on all sides and pinned down by mortars, rockets and mines. Eleven tanks were destroyed and the rest partially damaged and Israel lost at least 12 soldiers.

As unlikely as the Israelis might be to repeat these mistakes, they must figure out how to get their heavy armour past the Hezbollah teams that still lurk in the hills and valleys in the next round of fighting, if and when it comes.